The present invention relates to a device for controlling valve movements in an internal combustion engine which has at least one inlet valve and at least one exhaust valve for controlling the connection between a combustion chamber in the cylinder and an intake system or an exhaust system, a rotating camshaft having a cam curve, comprising a rising ramp and a falling ramp, being designed to interact with a valve mechanism for operating the inlet or exhaust valve by means of a valve spring, the valve mechanism comprising timing gear which allows the return movement of the valve mechanism to be controlled during the closing phase of the inlet or exhaust valve, independently of the falling ramp of the cam curve.
In internal combustion engines for vehicles it is sometimes desirable to be able to switch between different operating modes. For example, it is possible to switch between a conventional symmetrical cycle and an asymmetrical cycle, such as a so-called Miller cycle, by varying the inlet valve closing point during the engine induction stroke. The advantage of being able to switch between these different operating modes lies, for example, in the fact that the effective compression ratio of the engine can be varied in order to optimize the efficiency and to minimize the exhaust emissions. To this end, variable valve timing is therefore necessary. An example of such a known arrangement is demonstrated by SE523622. The device according to SE523622 allows the return movement of the valve mechanism to be controlled during the closing phase of the inlet or exhaust valve, independently of the falling ramp of the cam curve. By means of a hydraulically adjustable device which counteracts the closing force of the valve spring, the timing gear serves to retard the valve closure for an adjustable time interval. When running in the Miller cycle mode the device according to SE523622 functions as a conventional mechanical valve system, that is to say the greater part of the energy stored in the valve spring during opening of the valve is recovered when the valve returns to its closed position, in that the force of the valve spring imparts a drive torque to the camshaft during closure of the valve. When the arrangement according to SE523622 is not being run in the Miller cycle mode, the valve closure is retarded for an adjustable time interval by the closing force of the valve spring counteracting the hydraulic device. This means that the energy from the vehicle engine, which via the camshaft and the rocker arm is expended in tensioning the valve springs (during opening of the valve), is in valve closure dissipated into the hydraulic device, when the latter is connected for the purpose of retarding the valve closure.
U.S. Pat. No. 6,439,195 discloses the arrangement of a spring 82 between a fixed point on the engine and the rocker arm, with the aim of ensuring continuous contact between the rocker arm roller and its opposing camshaft cam. The force of the spring 82 and the opposing force exerted by the camshaft produce a vertically upward force on the rocker arm (see FIG. 1 in U.S. Pat. No. 6,439,195). This means that the rocker arm shaft 4 and the rocker arm 2 are in constant contact over a limited surface in principle situated at the very bottom of the circumference of the rocker arm shaft. Since the rocker arm, during a rotational cycle, turns only a few degrees forwards and back, it is more difficult for a lubricating film to build up on said contact surface (compared to a shaft which turns many revolutions). The lubrication problem with this design construction can occur over the entire cam curve, that is to say both when the rocker arm roller follows the base circle of the camshaft and also when the rocker arm roller follows the shape of the cam. The limited contact surface may further give rise to a concentrated wear, as a result of which the constituent parts may have a more limited service life. The location of the spring 82 in the arrangement according to U.S. Pat. No. 6,439,195 also makes both assembly and maintenance work on said spring more difficult.
It is desirable in an arrangement according to SE523622 therefore to maximize the recovery of the energy which is needed to tension the valve spring and to ensure lubrication of the rocker arm shaft/the rocker arm bearing and to prolong the service life of the constituent parts. It is desirable to facilitate the assembly of and any maintenance work on the valve arrangement.
The device according to an aspect of the invention comprises a device for controlling valve movements in an internal combustion engine which has at least one inlet valve and at least one exhaust valve for controlling the connection between a combustion chamber in the cylinder and an intake system or an exhaust system, a rotating camshaft having a cam curve, comprising a rising ramp and a falling ramp, being designed, via a rocker arm, to interact with a valve mechanism for operating the inlet or exhaust valve by means of a valve spring. The valve mechanism comprises timing gear, which allows the return movement of the valve mechanism to be controlled during the closing phase of the inlet or exhaust valve, independently of the falling ramp of the cam curve, and in which timing gear is designed so that the closure of the valve can be retarded for an adjustable time interval by means of a hydraulically adjustable force counteracting the closing force of the valve spring. The rocker arm and the valve mechanism are separated during the period of retardation, so that there is no interchange of force between them. An aspect of the invention is characterized in that a resilient element is arranged between the rocker arm and a fixed point on the internal combustion engine in such a way that the rocker arm continuously follows the cam curve as the camshaft rotates, and that the valve spring force is adjusted to the moving mass of the valve mechanism and to forces counteracting the valve closure, and that the force exerted by the resilient element is adjusted to the mass of the rocker arm, for the purpose of minimizing the energy losses of the valve mechanism.
One advantage obtained with the device according to an aspect of the invention is that the energy losses of the valve mechanism are minimized. It furthermore ensures that the contact between the rocker arm roller/sliding contact and the camshaft cam are maintained. Further advantages obtained are a more favorable pressure increase and reduced pressure fluctuation in the system hydraulics.
In one advantageous embodiment of the device according to an aspect of the invention the valve spring force is minimized and the spring force exerted by the resilient element is maximized. The advantage of this is that the energy losses of the valve mechanism can be further minimized.
In a further advantageous embodiment of the invention the sum of the spring forces is greater than what is required to counteract the force exerted by the sum of the moving mass of said valve mechanisms, the forces counteracting the valve closure and the mass of said rocker arm. This contributes further to a reduced energy loss.
Contact surfaces are formed between the rocker arm shaft and the rocker arm in order to maintain a balance of forces in the respective state of the valve arrangement. A first and a second contact surface respectively are formed between said rocker arm and rocker arm shaft during the period when the rocker arm and the valve mechanism are separated from one another and during the period when the rocker arm and the valve mechanism are not separated and exert a force on one another. In a further embodiment of the device according to the invention said first and second contact surfaces are displaced from one another. This serves to ensure that all slide surfaces of the rocker arm shaft and the rocker arm which come into contact with one another receive lubrication. The fact that the contact surfaces can be changed means that a better distribution of force is achieved over the slide surfaces. Any wear is more evenly distributed, thereby increasing the service life of the constituent parts.
In yet another embodiment of the device according to an aspect of the invention a force exerted by the resilient element and an opposing force in a contact surface between the rocker arm roller and the camshaft are directed basically straight against one another, so that the force acting on the rocker arm is minimized. In this way the contact surfaces for the two states can be subjected to the maximal displacement from one another (the central point for the contact surfaces is displaced by 180 degrees), which helps to maximize the distribution of force and to improve the lubrication of the rocker arm shaft/the rocker arm bearing.
In order to facilitate assembly and maintenance, in a further embodiment of the device according to an aspect of the invention said resilient element is arranged on top of the rocker arm.